The present invention is directed to radio transmission. It finds particular, although not exclusive, application to cellular telephones.
In order to achieve more-efficient use of the spectrum allocated to cellular-telephone use, the industry is moving to digital transmission, which facilitates time-division multiplexing of cellular-telephone signals. While this approach clearly provides the potential for significant improvement in channel capacity, the unavoidable transition period during which the previous, analog-frequency-modulation approach and the proposed, digital-phase-modulation approach must both be used has necessitated some equipment complexity. For example, base stations will need to interrogate the user equipment to determine whether it operates under the previous standard or the new one, and the base station must adapt to that standard.
Conversely, since not all base stations will immediately have the capability to operate under the new standard, subscriber terminal equipment that can operate under the new standard will also have to be able to operate under the old standard. Since the analog frequency-modulation mode is often embodied in voltage-controlled-oscillator circuitry while the typical new-standard equipment comprises a vector modulator, providing both capabilities can necessitate a costly duplication of circuitry. This cost results not only from the circuitry duplication but also from perpetration of the already prevalent expense of individually adjusting analog frequency-modulation circuitry during manufacture.